silverstripe-userforms/code/modifiers/DisambiguationSegmentFieldModifier.php

<?php

use SilverStripe\Forms\SegmentFieldModifier\AbstractSegmentFieldModifier;

class DisambiguationSegmentFieldModifier extends AbstractSegmentFieldModifier {
	/**
	 * @inheritdoc
	 *
	 * @param string $value
	 *
	 * @return string
	 */
	public function getPreview($value) {
		if($this->form instanceof Form && $record = $this->form->getRecord()) {
			$parent = $record->Parent();

			$try = $value;

			$sibling = EditableformField::get()
				->filter('ParentID', $parent->ID)
				->filter('Name', $try)
				->where('"ID" != ' . $record->ID)
				->first();

			$counter = 1;

			while($sibling !== null) {
				$try = $value . '_' . $counter++;

				$sibling = EditableformField::get()
					->filter('ParentID', $parent->ID)
					->filter('Name', $try)
					->first();
			}

			if ($try !== $value) {
				return $try;
			}
		}

		return $value;
	}

	/**
	 * @inheritdoc
	 *
	 * @param string $value
	 *
	 * @return string
	 */
	public function getSuggestion($value) {
		return $this->getPreview($value);
	}
}